server runing at just over 2.gb

Hi all,
My server is running daily at 2gb out of 16gb,when it gets smaller i have to delete files to get space.My boss said he is not going buy any we have to delete files that people are not using.

is there any to ask the server to delete files that has not been modified for at least a year.

I operate a LAN (domain)with over 50 users

thanks for your help